1. Field of the Invention
This invention relates to gas turbine engines, and more particularly to a means for mounting a sound absorbing structure to the engine case of such an engine.
2. Description of the Prior Art
A gas turbine engine has a compression section, a combustion section and a turbine section. The compression section has a rotor assembly and a stator assembly. An annular flowpath for working medium gases extends axially through the compression section. The interaction of rotor and stator components with the medium gases generates acoustic vibrations or noise.
In modern engines, sound absorbing structures typically face the working medium flowpath to absorb these acoustic vibrations and decrease the level of noise. One such construction is illustrated in U.S. Pat. No. 3,735,593 to Howell entitled, "Ducted Fans As Used In Gas Turbine Engines Of The Type Known As Fan Jets." In this construction, silencing material, such as perforated panels or porous sheets backed by honeycomb structures, are installed in the engine at various locations.
It is very important that these panels be durable and have an adequate fatigue life. Delamination of the panels and subsequent passage of the delaminated material downstream may cause severe damage to the engine. One cause of delamination is fatigue failure as repeated periodic stresses from acoustically induced mechanical vibrations are absorbed in the panels. Another source of periodic stress in the liner is engine case vibration. The vibrations in the case are particularly severe when the rotor blades successively strike the engine case. Such a strike may unavoidably occur during rapid accelerations or when the rotor assembly is subjected to gyroscopic maneuver loads.
Other sources of structural failure in the liner are not connected with vibrations. One such failure mode may occur during operation of the engine in environments where hail, mist, rain, sleet or snow are present. Water in its varied forms enters the engine along with the working medium gases and may be trapped in the honeycomb panels. As the engine ascends to higher altitudes the water turns to ice and may cause structural damage to the liner. Early fatigue failure of the panels may result and even delamination of the panels can occur with subsequent severe damage to engine components.
In response to the concerns expressed above, scientists and engineers seek to develop effective mounting structures that preserve the structural integrity of liners, in manners consonant with good acoustic performance.